This invention relates to an improved process for making cold reduced Al-stabilized steel, and more particularly to such a process of producing an Al-stabilized steel having high drawability and non-aging property.
High drawability and non-aging property are required for press forming operations. In the prior art, many attempts have been made to obtain improvements in both properties. None has been successful. For example, two stage cold reducing of a rimmed steel, or degassing of a Ti (Titanium) or Al (Aluminum) stabilized steel has been used.
The former steel may display high drawability in a rimmed steel, but, it also displays remarkably bad aging property. Thus, de-nitrizing process is used to avoid the aging. But, this also increases manufacturing costs, and hence, is unsuitable as an alternative.
On the other hand, the latter steels have good stability insofar as aging is concerned, but, the Ti-stabilized steel is expensive since it requires degassing and since such degassing decreases the surface quality.
The surface quality of Al-stabilized steel is superior to that of the above Ti-stabilized steel. But, the drawability of prior art Al-stabilized steel is substantially inferior to that of the rimmed steel made by a two stage cold reducing process. The problem in the prior art was thusly, to raise the drawability of Al-stabilized steel. Such an improved steel would be the best to employ in a press forming process.
Many attempts have been made to resolve this problem. None has succeeded. For example, the two stage cold reducing process previously used for rimmed steel was also employed in an attempt to improve the Al-stabilized steel. The effect produced in rimmed steel was, however, not obtained. The reason seems to be that the second cold reducing step and successive softening annealing step do not improve the drawability because precipitation of AlN has been finished at the intermediate annealing step.
Thus, it is recognized by workers in the art, that at the present state of the art, the most suitable steel for severe cold forming has not yet found.